Working IDE hard drives are getting harder to find. Compact Flash cards, the easiest modern substitute, aren’t all that easy to find anymore either. That got me looking at SD to IDE adapters, which convert cheap, readily available SD cards to a legacy IDE interface. This is convenient, but how’s the performance?
It turns out there are three limiting factors in SD to IDE performance: Card speed, adapter speed, and IDE bus speed. But since seek times on SD cards is lower, you can still see a performance improvement over a mechanical drive even if the transfer rates are disappointing. This is especially true of legacy systems that don’t have pre-emptive multitasking.
The appeal of SD to IDE adapters
You can get an SD to IDE adapter for around $10 online, and you can buy an SD card almost anywhere, assuming you don’t have an unused card laying around in a drawer somewhere. Low-capacity SD cards frequently go on sale for $5 or less, especially on or around holiday weekends. This is more convenient than the old method of using compact flash, as those cards aren’t nearly as readily available anymore.
This means for less than $20, you can have a silent, reliable replacement for an IDE hard drive. Just make sure you get the appropriate type. Laptops use a 44-pin card, while desktops and towers use a 40-pin card. The result is an IDE storage solution that’s reliable, silent, and generates minimal heat.
But how’s the performance? I’ve heard retro computer enthusiasts say these types of adapters provide exactly the same performance as an IDE hard drive. Is this the case, and why or why not?
The limiting factors in SD to IDE performance
As I stated before, there are three limiting factors in SD to IDE performance. Depending on how recent the system is, one or more of these can limit the performance you can expect from one of these adapters.
The SD to IDE adapter itself had a limiting factor that can affect newer systems. The adapter itself has a limit of about 25 megabytes per second. This is significantly lower than the maximum transfer rate of UDMA/33, which dates to the late 90s. We’re talking Pentium II-era.
Keep in mind hard drives of that era could only transfer 33 megabytes per second from their internal cache. As soon as they had to actually touch the disk surface, transfer rates fell off significantly. So a modern SD card can outperform a UDMA/33 hard drive since it can sustain that maximum transfer rate for a longer period of time. But don’t expect performance comparable to any reasonably recent SSD.
Some SD cards, particularly older SD cards, can’t sustain anywhere near 25 megabytes per second. Those older 1 GB SD cards you may have laying around in a drawer somewhere may only be capable of 10 megabytes per second, or less.
If you want good performance, you’re probably better off buying a newer, name-brand SD card, even if it means you don’t use the full capacity of the card. While even the cheapest $3.49 card I could find near me promises rates of 80 MB per second, a Sandisk or Lexar card will usually provide more consistent speeds, and better write speeds. Even some modern cards (as of 2019) tend to write files at rates of less than 25 megabytes per second, even if they can read at a much higher rate than that.
As time goes on, this will become less of a limiting factor.
IDE bus speed
Finally, there’s the speed of your IDE bus itself. Old-school parallel IDE has several transfer rates. Here’s approximately the kind of speed you can expect from your IDE bus depending on the age or generation of your machine.
|Mode||Maximum transfer rate (MB/s)||Approximate timeframe||Generation|
|PIO Mode 0||3.3||Pre-1995||286|
|PIO Mode 1||5.2||Pre-1995||386|
|PIO Mode 2||8.3||Pre-1995||486|
|PIO Mode 3||11.1||1996||Early Pentiums|
|PIO Mode 4||16.7||1996||Late Pentiums|
|UDMA/133||133||2001||Pentium 4 and newer|
What this means is that if you put one of these adapters with a fairly new card in something older than a Pentium II, the adapter isn’t the limiting factor. The write speed of the card, for that matter, may not be the limiting factor either, especially in a 486 or older computer.
There’s one more limiting factor, but it has nothing to do with speed: your capacity limitations of your system BIOS. Here are some ways to deal with those limitations.
Comparing SD card speed to traditional hard drive speed
Benchmark data for pre-2000 hard drives is getting hard to find. In some cases the sites are still active but the graphical benchmark data may not still be available. But mainstream hard drives from Quantum and Maxtor around mid-2000 tended to transfer data at 15-20 megabytes per second on average. UDMA-133 drives from 2003 can transfer data faster than 25 megabytes per second.
So if you’re replacing a drive from before 2003, an SD to IDE adapter will probably outperform it. It becomes a sketchier proposition in PCs newer than that.
In my own experience, I’ve found the transfer rate from an SD adapter to be about 25% faster than any 1990s hard drive I’ve tried in my old 486. The bus is the limiting factor.
One more factor to consider in performance
But there’s more to performance than transfer rate. There’s also seek times. Even on a drive from 2003, you can expect a seek time of greater than 9 milliseconds. The seek time on even a really old compact flash card will be better than that. On a modern compact flash card, you can get a seek time of 1 millisecond or lower. The seek times are so short most DOS benchmarks get confused.
Seek time is the reason we defragmented hard drives. When your seek time is around a millisecond, the need to defragment a drive essentially goes away.
The overall performance effect of an SD to IDE adapter
If you replace an IDE hard drive with an SD to IDE adapter, don’t expect the kind of improvement you see with an SSD in a modern computer. The performance won’t be any worse, especially in a pre-P4 machine, but there’s usually too much overhead involved in older machines for you to get the 2-3x improvement you see with an SSD in a modern PC. The improvement in seek times is nice, but a disk cache lowers the impact.
That’s why some enthusiasts say solid-state solutions provide exactly the same performance as an IDE hard drive. If you’re using a disk cache, which you probably will be, the difference isn’t great, especially with DOS software.