A 6TB Premiere is possible to create but it is not all easy. The biggest obstacles are converting the TiVo APM all to 64bit entries and a manual coalesce. This will not work on a Roamio as TiVo changed how many partitions can exist on the main drive. Anything over 14 partitions causes a reformat of the drive. This is current as of 20.4.7a.
To create the 6TB image use MFSTools 3.2. The basic steps are as follows:
1) Use MFSTools 3.2 to copy your current image to a 6TB but limit the image size creation to 4TB. The current MFSTools cannot handle an image that has had either partition 10 and/or 12 coalesced previously. There is a mod that is being tested and seems to work if only partition 12 had been coalesced.
2) Use MFSTools 3.2 to add a pair of partitions to the newly created image.
3) Convert all APM entries except for the first one to 64bit entries.
4) Coalesce the last pair of partitions (15 and 16 into 15)
5) Modify each entry in the APM to reflect 15 total partitions.
6) Put it in your Premiere and let it boot and divorce.
You then should have a 6TB Premiere and if you used the appropriate switches in the first step, still have all recordings.
Our current coalescing tools only work on 32bit APMs so these tools will need to be either modified or new tools will need to made. Will also need to have a tool that will convert an APM from 32bit to 64bit.
I'll try getting a write up together with more details but hopefully someone will help develop those tools to make this process easier and much less prone to errors.