I have a PC with 4 harddisks and one ssd drive, presently PC boots from the 1st harddisk and other harddisks (sometimes 1, 2 or 3, depends upon the requirement) are used for the storage.
From this I get the impression that you remove
disks from your configuration depending on your storage need?
Am I right?
If you remove disks, you will find that you change the definition of some of those left. (The sda, sdb, sdc definition)
With 4 disks I would assume (My first mistake! you shouldn't assume anything!) you have a couple of PATA controllers on board, both with a master/slave disk? Your fifth disk possibly a SATA connection? You need to let us know how they're connected.
Then... The boot device is usually set up in the BIOS (Sometimes it's just set as an adapter and the adapter's BIOS defines the boot device.) Too many things you haven't told us yet!
Grub also uses the sda, sdb, sdc definitions to boot the required image so if you remove a drive, it may well find the wrong image.
Can you clarify exactly what you're doing so I don't have to assume anything 'cos my head hurts.