Why does sandisk refer to this as solid state when it is really just a fast flash drive, and why the critical reviews?

SanDisk refers to this as a solid state drive because it is a solid state drive in a flash drive form factor over a USB 3.1 Gen 1 interface. This device uses a SSD controller over an SCSI-to-USB bridge. A typical flash drive uses an eMMC controller, which optimistically has similar peak read speeds but falls flat with writes, complex workloads, and extended continuous use. For an example of this distinction in practice, a USB SCSI device such as this can be configured for direct access by a Windows Hyper-V hypervisor. Most USB SSD devices support TRIM and all that I'm aware of support SMART. As a cautionary note, there are some products that support flash drives that won't support this device or similar. For instance, I once attempted to use a similar device with a printer/copier that could print from flash drives, but the printer didn't support USB SCSI and failed to mount the drive (formatted with a single exFAT partition). It was able to print from an exFAT traditional flash drive.

Since this usb is 3.1, will the speed be less if I'm using it on the usb 3.0 port on my pc? If yes, by how much?

Short answer - No. Per USB.org, the terms "USB 3.0" and "USB 3.1 Generation 1" are "synonymous" for the same 2008 technology. The change from "USB 3.0" to "3.1 Gen 1" was just a (very confusing) marketing/licensing rebranding issue. So USB 3.0 = USB 3.1 Gen 1 = max 5 Gigabits per second (Gbps) = 625 Megabytes per second (MBps). Simple ... until 2013 when they introduced the next generation technology at double the speed as "USB 3.1 Gen 2" = max 10 Gbps. Note the "Gen 2". So now if you want the older 5 Gbps, look for "USB 3.0" or "USB 3.1 Gen 1" or just plain "USB 3.1". But if your PC supports it and you want to make sure you're getting the new 10 Gbps technology, absolutely look for the additional "Gen 2" or don't buy it.

How does the slider on top of this usb stick lock? Do you have to press the slider down or something to move it up and down?

You can extend the connector whether or not you press down on the button. Once extended, the connector locks in place requiring you to press down on the button to retract it. However, it's pretty easy to accidentally press down on the button while inserting the drive to your computer or usb hub. So you must avoid that if you want to insert it easily. Hold it along the edges or wrap your finger across the back and down the side and you can insert it easily.

Does this need to be reformatted to store a large single file greater than 4gb

Actually the 256GB arrived exFAT, allocation unit size 1024k, probably ideal for large files such as video. The down side is that if you try to store a large number of files, such as 245,000 jpg that are about 130GB total on my computer, Windows 10 would report not enough space! The simple solution is to reformat in exFAT with an allocation unit size of 128k or less. Then everything fits since not so much wasted space of each file.

Read Speed	420 Megabytes Per Second
Connectivity Technology	USB
Color	black
Model Name	Susbie4837
Flash Memory Type	USB
UPC	619659152512
Global Trade Identification Number	00619659152512
Size	128 GB
Color Name	black
Batteries Included	No
Batteries Required	No
Item Model Number	SDCZ880-128G-G46
Model Year	2017
Included Components	SanDisk Extreme PRO 128GB USB 3.2 Solid State Flash Drive
Special Features	3.2 Generation
Product Dimensions	7.09 x 2.13 x 1.14 cm; 18.14 g
Item Weight	18 g

128GB SanDisk サンディスク USBメモリー ExtremePro USB3.1(Gen 1)対応 R:420MB/s W380MB/s スライド式 海外リテール SDCZ880-128G-G46

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128GB SanDisk サンディスク USBメモリー ExtremePro USB3.1(Gen 1)対応 R:420MB/s W380MB/s スライド式海外リテール SDCZ880-128G-G46