Can I connect my 100GB 3.5 IDE drive to a Soekris box
From Soekris Info Wiki
Can I connect my 1000GB 3.5" IDE drive to a Soekris box?
In short: No. Or at least, it won't be easy. Even if you have a suitable adapter for the 44-pins connector, you would have to have the 3.5" HDD draw power from an external source, as the Soekris boards power supply is limited.
Option 1 - USB. You can connect a 3.5" IDE (or SATA) drive using an external enclosure with a USB interface and connect it via USB (for those soekris models with USB support.) Heck, get a good hub and connect lots of 3.5" drives! Be sure to use an enclosure with its own power supply unless you're certain you can supply enough power via the USB bus. Performance will likely suffer but you'll have storage.
Option 2 - SATA. For the NET5501 only, you can connect low-power 3.5" SATA drives (like the Western Digital GreenPower series) using a special power supply cable (for example from www.KD85.com) from the NET5501 power header to the SATA drive. You will need a good 12 volt power supply (say 25 watts) for the NET5501, and you'll need a large enough case, perhaps with a cooling fan. The WDC WD1000FYPS 1000GB drive is known to work like this - it runs happily with no need for a fan in a 19-inch rack case - but your mileage may vary with other drives: so you might want to ask on the Soekris mailing list before buying such a drive.