My understanding is that the 1 TB limit still applies.
So, you could use a 4 TB disk with a 1 TB partition on the front. That would in fact be very fast, as the front ~half of the disk is the fastest area.
However while you could put a 3 TB partition using the rest of the disk, be aware that if this is accessed at the same time as the other partition then the heads will spend most of their time seeking backwards/forwards from one end of the disk to the other. Performance will plummet, even with a 7K4000.
For a speedy system the best compromise is probably to get a 2 TB disk and leave the 2nd half unused. It may feel like a waste, but it has a definite speed advantage. Same goes for a 7K4000 and using it as a 2 TB disk. "Buy drives twice as large as you need" is a good performance guideline.
My advice: easiest to go with a new 1 TB drive. Depending on the model you get it will hopefully be quieter and faster than the old one. You might still have to deal with the 4K sector boot issue though.
Actually, is the failing drive under warranty? What are you planning to do with the replacement drive they send you? Maybe get a 4 TB drive, put a 1 TB partition on the front and copy your existing drive over before it dies. Boot off that for a while. Then when the replacement drive comes back copy the boot partition back to it, have both drives in the system, and reformat the 7K4000 as one big data drive.
However, do you really want a boot drive as large as 1 TB? It does complicate backup and restore strategies (OK, so Fusion is pushing people in that direction too, but still...). In our cases we usually have more freedom to add spindles than an iMac/mini/MacBook user.
On lammergeier I have a 128 GB boot drive, and have a 128 GB clone copy of it at the tail end of a 750 GB drive. But the other drives are whole partitions.
On FractalPro I have a 240 GB boot drive, and a 241 GB clone copy on another drive. The performance of the clone copy is usually not critical, so I put it at the end of a drive. Unless that drive is larger than 1 TB, in which case I have to compromise and put it at the start of the drive (rather than dividing the rest of the disk into two separate areas).
