1. You need to put the spare drive in a windows machine, use diskPart and clean the disk.
2. Use a new USB stick, download the old boot loader and DSM version that was originally on your NAS and write that to your USB drive.
3. Remove all the SATA cables from your DSM box for all the hard drives in it.
4. Plug in your spare clean drive and verify the SATA cable used is physically attached to PORT1 on your motherboard.
5. Plug in the new usb stick with the old boot loader and DSM version.
5. Boot into DSM via your USB and install like normal.
6. Once install Is successful, shutdown.
7. Install the SATA cables into all the old existing drives, MAKE SURE TO KEEP SATA 1 PORT Cable plugged into the SPARE Drive you used. The rest of the original NAS drives need to be plugged into 2,3,4,5,6 SATA ports etc.
8. Once all SATA ports are connected back, boot into DSM. Go to the disk Management section and rebuild your RAID arrays.
9. Your Data should now be accessible again. You will need to re-install some of your old packages, but no big deal. Shutdown after Raid arrays have completed.
10. Disconnect the Spare Drive from SATA 1 and plug in your old/original existing drives into SATA PORT 1-6. IE: If you have a raid 1 array, SATA port 1 and 2 would be for RAID 1. 2345 would be used for RAID 5. Not sure how your RAID arrays are configured...
11. Start your DSM box up and it should be back to normal.
I did this same thing about 2 days ago due to a borked upgrade. Worked flawlessly.