One important rule to adhere to with all memory cards is to not insert or remove one in your 100LX while the power is on. The built-in applications are constantly communicating with the card, whether you're actively entering data or not. It is possible to lose data if the card is removed while this is happening.
SRAM Memory Cards:
SRAM memory cards are completely compatible and interchangeable between the 100LX and 95LX. However, do not reboot your 100 lx within 2 seconds of turning it on or inserting a card. There is a card sizing routine that performs test writes to the card whenever it is inserted with the machine on or the 100LX turned on with a card in the slot. If this is interrupted by pressing either (CTRL)-(ALT)-(DEL) or (<Shift>)-(CTRL)-(ON), the original bytes will not be restored and the card will be corrupted. It will also make the card unreadable on a 95LX.
DoubleCards:
ACE DoubleCards will work fine on the 100LX, providing the software needed to read the card is updated. You must have ACEDRV.SYS, version 2.12 installed on your 100LX. This driver is available from ACE directly or on CompuServe in Library 14 of HP Handhelds forum. If your DoubleCard has a SHELL=C:\COMMAND.COM /P statement and you wish to make it bootable on both a 95LX and 100LX, you should edit it to read SHELL=COMMAND /P. (Make sure there is a space before the "/.") You will not be able to see the uncompressed portion of the card on the 100LX as you can by going to B: drive on the 95LX.
SunDisk SDPL-5 FlashRAM/HP95 Flash Disk Cards:
SunDisk FlashRAM cards designed for the 95LX may be used on the 100LX, providing these steps are taken. Never insert a SunDisk HP95 card into a 100 with it turned on. This will cause it to reboot and all unsaved data will be lost. Set up your CONFIG.SYS on C: drive of the 100LX without using SDPDRV.SYS (the 100LX does not need it). I would also recommend using rechargeable NiCds in your 100LX as they are better suited to handle the higher current drain when writing to the card. These cards are significantly slower to write to than SRAM cards but are of higher capacity. The provided Stacker software is compatible on both machines.
SunDisk SDP-12 FlashRAM/HP100 Flash Disk Cards
The 12v SunDisk FlashRAM cards may only be used in the 100LX. Do not insert a 12v card into a 95LX. The 100LX does not require software to read these cards. The 12v cards are about three times faster than the 5v ones, but this comes at a cost of much higher current consumption. NiCd batteries are the optimum choice here.
We recommend using Stacker on either of the FlashRAM cards, especially if you store a lot of small files. Any disk or memory card is divided up into minimum storage units called "clusters." On the FlashRAM cards, the cluster size is 4K. That means that a small, 40 byte file is really taking up 4,000 bytes. If you use Stacker on the card, the cluster size is 0.5K.
In summary, if you need the most disk space for the dollar and don't mind slower transfer speeds, the FlashRAM cards are an excellent choice. SRAM cards are the choice if speed is your top priority. However, the 12v FlashRAM cards are closing fast.