SDHC memory cards is max 32GB. If you want more, you have to wait for a player that supports SDXC cards, those can go up to 2TB, but it wil take years before they are affordable and available.
If external hard drive support is very important, then maybe a player with his own power is much better, i have the Diyomate S9, i buyd this because it has more codec support then Nationite HDVP-2 for example. Also, the Diyomate S9 has the REALTEK 1073DD+ wich is very good, also, you can connect an external hard drive right at the SATA connection on the player, it supports both a mobile 2.5 and a external 3.5 hard drive, with NTFS support.
If you realy need a portable player, then just an external hard drive with it's own power connected trough USB OTG should work fine, however, you always have to look out that there is enough battery life for something you gonna watch. If you want a device that is always connected to your TV, always look for one with the REALTEK 1073DD+, if you want a portable device then go for a SoChip SC9800 chip, because those 2 for this type of players are the best at the moment. I had several other devices with previous chips, none of them was good, only at this moment those 2 chips are good in my view. The chip that is inside the player is one of the most important things, second, all you realy want is the 2th important.
The best would be to take a pmp, and a player that is always connected to your tv, then you have all you need
, but if you get a player with another chip then those 2, you gonna look for another player i think because it doesn't play the files good or the codec support is bad. The cable that you are looking for is a "Y USB Cable", you put the mini-usb in the mobile hard drive, one usb end goes into a usb charger, and the other end goes into the USB OTG cable that is connected to the player. Several users where able to format the internal memory of an ainol player with the SC9800 chip, but i'm not sure it is safe to when you want to put a new firmware on the player in the future, it could be that you have to reformat it to FAT, apply the firmware, and format it again to NTFS, because the firmware can't detect the NTFS file format, but i'm not sure of that.
Thanks for your really detailed response to my queries and situation - I forgot to thank you for taking the time to do so.
Regarding the "Y USB cable" you refer to, I just wrote about this issue in my response to maderin1 (also a very helpful forumite, whose advice is also most gratefully received) - I don't think it will help with the Ainol v6000HDT because the input on that player is mini USB. The USB OTG cable supplied is simply a USB mini male out to USB A male type (so bog-standard to connect straight to a PC USB input).
I'm absolutely knackered without resorting to soldering and the like - something I would have no chance of carrying out successfully, unless you know of any other cables or adaptors that might help.
I've searched through masses of USB lead websites but I'm yet to find anything suitable, and I worry that some of the combinations aren't produced because they are a hazard to either or both devices, which makes me even more reluctant to simply stitch together a USB mini male out to another USB mini male, which is essentially what I need in this case.