I use an external SCSI drive on my A2000 and A3000 and a 5.25" internal in my A1200 Tower.
If anyone is interested, I'll be selling 3 x 5.25" IDE ZIP 100 drives (ideal for mounting in an A1200 Tower or a PC case) on eBay at the end of the month (along with an A1200T with a Power Flyer Gold and internal ZIP 100 fitted).