Hi all,
I have always used Slackware exclusively on my laptop, so installation has always been straightforward.

I installed Slack a while ago on my work PC to a separate drive, but it overwrote the UEFI for my Windows drive, not the same drive as I installed Slack to.

I'm wanting to replace Debian on my home PC with Slack, but am cautious as I have a similar setup(Windows on one drive, linux on the other).

At present, I have GRUB2 EFI on my second drive with my Linux install, and the Windows UEFI bootloader is untouched on the first drive.

Before I go messing with my setup, is there a way to ensure during Slack setup that it only installs EFI data to the drive on which I am installing it?

Sorry, I would just go and try it, but I rely on this PC for work at home.

you could unplug the windows drive before installing debian but not sure how that will effect the ability to boot either one.

As a last resort I could do this to install Slack. Should work, just means I'd have to use the system boot menu to pick a boot drive.

Worse than that, you'll have 2 EFI partitions. I don't imagine that would lead to good things, but I haven't tested it myself.
That advice worked for MBR, but I wouldn't do it on UEFI with gpt.

I suspect you didn't over-write anything, but merely added a new entry to the NVRAM on the motherboard, which became the (new) default boot option.

It works with an external usb drive, or at least it does on my asus laptop anyways, not sure with two internal drives.

I can confirm that having an EFI partition on both internal drives works just fine. It's what I have at the moment.

The issue I had last time was that Slack automatically detected my existing Windows EFI partition on the other drive and installed ELILO to that.

For some value of "just fine".
I have never had any issue using a (Windows) pre-allocated EFI for Linux - grub[2] only in my particular usage.

README_UEFI.TXT is very useful as a first reading in these cases.

The Slackware installer (64-bit) does this:

• Make a list of all EFI partitions on your system.
• Take the first one found initially (probably on /dev/sda, as it scans the devices beginning with this one in alphabetic order).
• But if there is one on the same device as the root partition, prefer this one if different
• Format this partition in FAT32 (if not already formatted).
• Prepare a record for it in /etc/fstab that will be written at end of installation.
• Later (during the CONFIGURE step) offer to skip running lilo and run elilo instead, if it detected that the firmware is in EFI mode and there is an EFI system partition set up (however you may install both, see below).
• Then if you agreed to run elilo it actually runs the script /usr/sbin/eliloconfig that put the stuff needed for booting in the EFI system partition.

Some notes:

• Theoretically (read: if the firmware complies to the UEFI specification) you can have several EFI system partitions, that the firmware will somehow "concatenate" to find the EFI images and applications.
• Conversely you can have as many EFI images or applications that you want in the same EFI system partition.
• You can and may choose to install both lilo and elilo. This doesn't hurt at all and allows you to boot in both EFI and legacy modes if you want.
• Especially in case of multi-boot it can be handy to write a boot entry for Slackware in the firmware's boot menu (alongside the existing ones), which eliloconfig will propose you to do.