Unfortunately I don't have good advice on specific screws to use. The ones mounted to my 3000's old hard drive (not sure if they are original since the machine is second-hand) are too wide for the adapter. In situations like these I usually just grab a few different kinds from the hardware store - they're cheap enough that I'm willing to sacrifice a few cents on some superfluous ones (can't remember exactly what I bought in this case). Also: while the original drive used 4 mounting holes, the adapter only needs 2.
I ended up backing up the whole drive to another Amiga, prepping and copying to the adapter/SD card using that same other Amiga, and then just plugging it into the 3000. But you've got the right idea:
- disconnect your secondary drive,
- plug in the adapter,
- prep that new SD drive (make sure to set the "bootable" flag on the boot partition!),
- copy everything to the new SD drive,
- then replace your boot drive with the adapter and reconnect your secondary drive.
Before/during this process you'll want to make sure the SCSI ID of the adapter is set to not conflict with either of your existing drives - that's what the USB port (and associated configuration utility) is for. (see
here)
I do wish these adapters had jumpers to set the SCSI ID - having to connect to another system to change the ID is a bit frustrating.
Now, if you want to get really fancy, these adapters actually allow you to emulate multiple SCSI devices. For instance: you've got a 16GB SD card and don't want to deal with all of the >4GB issues on a default OS2.x or 3.x system. The config utility can have the adapter present that one 16GB card as four 4GB devices using four different SCSI IDs (say, 0-3). You can also "hide" data beyond a certain point, i.e., presenting one 16GB card as one 4GB card.
If you want to get rid of your secondary hard drive, you could either copy/move its contents to another partition that you create on the SD card with HDToolbox, or you could copy/move its contents to another virtual device that you create with the USB config utility.