You can never have too much storage in your PVR. When I purchased my Topfield 7100+ PVR I chose a cheap model with a small 320GB hard disk with the intention of upgrading when the disk got full. It accepts any standard SATA 3.5" hard disk, of which 1 and 2 terabyte models are now the best value for money. Although I saved money by delaying my hard disk purchase until large disks got cheaper, and although the upgrade procedure was pretty straightforward, it was a very time consuming operation.
Although Topfield do not explicitly encourage users to replace their disks, it is implicit in their manual which states "If a new hard drive is installed, it must be formatted because you can neither record nor use time-shift with an unformatted hard disk". Replacing the disk does involve breaking a "Warranty void if removed" sticker, but in Australia these stickers do not override consumer rights.
The main problem in upgrading a full hard disk from a PVR is that you need to move all of your stored programs onto the new disk. For this you need to copy everything onto an intermediate disk, and then back onto the new disk. This PVR allows for the attaching of external hard disks via eSATA and USB. It also has an ethernet connection. I have an old laptop connected via ethernet which I use as a wireless bridge so my main PC can connect to the PVR. The speed of copying files from the PVR via these connections is:
external disk via USB = 19 GB / hour
laptop via ethernet = 26 GB / hour
PC via WiFi = 10 GB / hour
The PVR can ably multi-task, so it is happy to record four programs, display a pre-recorded program, and copy files to an external disk simultaneously.
On first glance, the external disk looks to be a winner. However, the standard Toppy interface only allows for the copying of a single file per operation. You can't select a bunch of files and get them all transferred, but have to manually initiate each operation. Luckily, an FTP server is also provided. Using an FTP client such as Filezilla you can select a batch of programs to move, and then leave it running overnight. I did this, copying the files via WiFi to my main PC. The old attached laptop only had a small disk, so was not an option. (I don't know of a way to copy to an external attached disk via FTP.) After a couple of days all the files were copied, and I was ready for the crucial next step, installing the new disk.
Old Topfield PVRs, such as the venerable Topfield 5000PRVt, were very picky about hard disk connections. If you used a slightly different PATA cable then it might refuse to work. The new TMS models are much improved. They use SATA, and don't appear fussy at all. Vibration dampening is used around the disk, and the screws to remove are obvious and easy. The new disk is easy to place and connect without much manual dexterity required. When the PVR is first turned on with the new disk it'll spend a few seconds formatting and then you're away. All of your timers are stored in flash, so they'll still be there. All you have to do is copy your old programs back from where they are stored.
As long as you schedule plenty of time for transferring of programs you'll have no trouble with upgrading the internal disk.