Is it safe to remove a partition and form a new one with already unallocated space?

"You'll need a specific software for that. There's a program from Acronis (Disk Director) which lets you move and resize partitions (it's payware, but worth it, saved my day a couple of times ... and moving 600Gig will take some time, expect a few hours operation).

But it will let you move the stuff on screen visually (much like the images you made) all beforehand, i.e. you reorganize without actually doing it, then you press 'go' and it will perform all the steps.

So what you'd do with that program, is delete partition a, move b/c/d to free space at the front, create new partition in now larger free space at the end.

Since this seems to be your secondary drive, not your drive with the operating system, I don't see much problems ahead (except the normal SNAFUs which can happen any time)."

"I dont want to lose 600 gigs of stuff

Regardless how safe and secure any method may be, you do not tamper with partition tables without a backup."