kevincjp said:

Regarding the Aspire 5 515-56-54KJ, there is an empty (unused) 2.5inch expansion bay. Can a Western Digital 1TB WD Blue 3D NAND Internal PC SSD - SATA III 6 Gb/s, 2.5"/7mm, Up to 560 MB/s - WDS100T2B0A, be installed as supplemental storage? If yes, does anyone know of a cap to storage? Is 1tb  (2.5"listed above) + 512gb (Sky Hynix HFM512GDJTNI-82A0A BA, NVMe, M.2 2280, - original main storage) for a total of 1.5tb good?

Your laptop hasent got a cap/storage limit, it comes with an OEM boot drive of capacity 512GB PCIe/NVMe M.2 drive and its formatted in GUID Partition Table (GPT) which the format will recognise a capacity of over 2TB plus capacities and this laptop has provisions for a secondary drive of the 2.5 inch SATA3 @ 6GB/sec type. Your Win-10 boot drive will always be formatted and run in the GUID Partition Table (GPT) format to its BIOS. as the older  Master Boot Record (MBR) type format drives and BIOS's are being replaced as its got limitations and its not as good as the GUID Partition Table (GPT) format. 

Just a short explanation on these drive formats, as i've mentioned above, MBR does have its limitations. For starters, MBR only works with disks up to 2 TB in size. MBR also only supports up to four primary partition, if you want more, you have to make one of your primary partitions an “extended partition” and create logical partitions inside it. This is a silly little hack and shouldn’t be necessary. While GPT doesn’t suffer from MBR’s limits. GPT-based drives can be much larger, with size limits dependent on the operating system and its file systems GPT also allows for a nearly unlimited number of partitions. Again, the limit here will be your operating system—Windows allows up to 128 partitions on a GPT drive, and you don’t have to create an extended partition to make them work.

If you want to add this 1TB WDS100T2B0A or any similar SATA3 6GB/sec drives then remember that you can use this as a slave drive in MBR format (with the linitations above) as its 1TB is not a greater capacity than 2TB.